Metal alloy especially adapted for use at high temperatures



atenied Mar. 6, 1951 METAL ALLOY ESPECIALLY ADAPTED FOR USE AT HIGH TEMPERATURES Francis B. Foley, Philadelphia, Pa., assignor to The Midvale Company, Philadelphia, Pa., a corporation of Delaware No Drawing. Application June 26, 1948, Serial No. 35,533

5 Claims. (Cl. 75-134) .My invention comprises a metal alloy for use at elevated temperatures such as are experienced in gas turbines and which is believed to excel other alloys heretofore made in the following respects:

1. It has a higher combination of creep strength, fracture time for a given load and temperature, fatigue strength and resistance to sealing at high temperatures. 7

2. Resistance to grain growth.

3. Ease of hot working.

Alloys intended and adapted for the above special use having compositions containing the same constituents as my improved alloy are known in the art. My improved alloy, however, differs from these known alloys in several essential respects. All such alloys contain a substantial percentage of molybdenum, which, for reasons hereinafter explained, must not be present in my alcentage of iron will hardly exceed ten per cent. With minimum permissible proportions of the other constituents the percentage of iron will not exceed about25 per cent. In fact, nickel, chromium, and iron are largely interchangeable if the percentages of nickel and chromium are maintained within the above limits' The presence of some iron is therefore desirable. It is also desirable from the standpoint of economy of manufacture and could hardly be practically excluded when, as is usual, certain alloying metals are, in commercial form, alloyed with iron.

.Manganese and silicon are not essential constituents, but their exclusion is difficult and their presencein small proportions does not impair the qualities characteristic of my improved alloy.

I have found it distinctly advantageous to keep the proportions of the several ingredients within the following ranges:

Mn Si Gr 00 Gb N Fe 18-22 27-32 5. -8 2. 5-4 bal.

loy in any substantial proportion and which is, decidedly preferably, excluded altogether. If present at all its permissible proportion should An alloy that is particularly acceptable for commercial use has the following approximate composition:

or 00 Wos N Fe Si Ni 29.25 6.30 I 3.50 bal.

ents, is as follows:

This composition, with little deviation from the proportions given, is distinctly preferred.

The main objection to carbon approximating 1.5% is that high carbon requires great care in heating and working. Even in forging one could, with difficulty, forge up to 1% of carbon and possibly up to near 1.5%. To avoid hot working difficulties and consequent uncertainty of results the percentage of carbon should not exceed .50 per cent and it is desirably considerably lower, while I 0 Mn Si Ni Cr 00 -W 'Cb N Fe up to 1.50 up to 1.5 up to 1.5 30/40 18/24 17/36 4/8 2/6 .10/.20 bal.

Iron is not essential. When the other constitucuts are present in preferred proportions the per.-

in forgings the carbon percentage should not exceed..30 per cent. It is also highly desirable to 3 maintain a proper carbon-cobalt balance, as hereinafter explained, which, in the case of a desirably high cobalt percentage, requires a comparatively low carbon content.

Vanadium in my composition is an equivalent of tungsten and may replace it in whole or in part, but in the ratio of one part of vanadium to two parts of tungsten.

While in most alloys tungsten and molybdenum function as equivalents they are not such in my composition. Molybdenum cannot be substituted even in part for tungsten and in my alloy is merely one of a number of elements which might be added in unsubstantial proportions without appreciable benefit and without pronounced harm.

There are two principal reasons for excluding molybdenum, or, to express it otherwise, molybdenum has two undesirable effects when a constituent of metallic alloyssubjected to elevated temperatures. One of the changes in structure at elevated temperatures which gives rise to embrittlement in alloys of the type covered by this invention is the formation of a non-magnetic very hard phase identical in crystalline structure with the brittle sigma phase found in iron-chrom l m a ir n e hremium alle s. Molybdenum is twice as active as tungsten in develop.- ing this brittle phase and I find tungsten n this alloy to be as effective as molybdenum in conferring resistance to deformation in the alloy,

' The other undesirable effect of molybdenum is in its lowering of the resistance of the metal to oxidation at high temperatures. Under certain conditions of exposure molybdenum can lead to the complete destruction of the, metal by its conversion to oxide in an extremely short period of time. Such rapid oxidation has been called catastrophic oxidation.

The use. of tungsten as a strengthener avoids h un s a l iee s a d omot s or eahil- 1 y.

The presence of nitrogen in amounts of .10 to 20% restrains grain growth.

It is generally recognized that coarse grain size promotes resistance to deformation at high temperatures and that a f ne grain structure is relatively weak at high temperatures. I find a i fine in. c a ac e s c of m al be taken advantage of in my alloy to render it more readil responsive to, deformation in hot working and that the coarse grain advantage in strengthening can be provided by means bf a heat treatment involving exposure of the'alloy' to a temperaturehigh enough to produce a desirable larger grain size. This property of the alloy of lending itself readily to hot working makes hot. forging, in either open or closed dies, and rolling, both practical and economical. Turbine blading sections have been produced by drop forging, and the alloy under such conditions of worke ing has been found to be extremely ductile and to display a degreeof forgeability greater than any of the currently used alloys designed for gas turbine blading. This was evident from'the abilrapid grainegrowth. My composition in the wrought conditions attains an A. S. T. M; grain fectively than one which undergoes relatively size of only 4 or 5 after exposure at 2375 F., which is the highest temperature to which it would be exposed for coarsening; higher temperature involving a risk of reaching the solidus of the alloy and thus producing physical damage and a weakening of the structure.

When melted to the preferred analysis this alloy will hold a load of 17,000 p. s. i. in tension at 1500 F. for 1000 hours and in fatigue at this temperature it withstands million reversals of a stress of 42,000 p. s. i. before failure occurs.

I find that the carbon content of the alloy has a great influence on the resistance of the metal to failure under load at elevated temperatures. Over the limits between 0.15% and 0.30%, carbon is more effective in strengthening the alloy than is cobalt within the range 17.0 to 30.0%. However, this advantage of carbon is limited by the increased difficulty with which the alloy is hot Worked as the carbon increases. Thus, for maintaining strength at high temperature, car bon may be increased as the cobalt is decreased,

' but strength thus obtained is at the sacrifice of hot worlging properties and such high carbon a1- loys are best suited for use as castings, whereas the high cobalt-low carbon combination is better suited to a wrought. product. The effect of carbon and cobalt is shown in the following table:

stresstiin p s. i

- pro uclng 0 N1 or G0 W Cb N failure in 1000 hours A. 22 33. O 20. 8 29. 5 7'. 2O 2. 92 13 16,900

preferred specific composition hereinbefore given.

What I claim and desire to protect by Letters Patent is:

l. A metal alloy especially useful as a composition for structures, such as gas turbines, normally subjected to high temperature consisting essentially of carbon not exceeding 1.50 percent, small percentages of manganese and silicon, nick- 1 e1 30 to 40 per cent, chromium 18 to 24 per cent,

cobalt 17 to 35 per cent, tungsten 4 to 8 per cent, molybdenum 0 to substantially less than one per cent., columbium over 2 and not over 6 per cent, nitrogen .10 to .20 per cent, the remainder-substantially all iron, the percentage of the ingredients other than iron being such, within the specified limits, that the proportion of iron does not exceed 25%.

2. A metal alloy especially useful as, a composition for structures, such as gas turbines, normally subjected to high temperatures consisting essentially of carbon .10 to. .30 per cent, small percentages of manganese and silicon, nicke .1 to 35 per cent, chromium 18 to 22 per cent, cobalt over 27 and not over 32 per cent, tungsten 5.5 to 8 per cent, molybdenum 0 to substantially less than one per cent, oolumbium. 2.5 to 4 per cent, nitrogen .10 to .20 per cent, the remainder substant llr all. iron, the percentage of theradien s other than. be g su'eh.. thi 1i the specified ranges, that the proportion of iron does not exceed 14%.

3. A metalalloy of the composition specified in sition for structures, such as gas turbines, normally subjected to high temperatures consisting essentially of the following constituents in approximately the following proportions: carbon .19-.23 per cent., manganese and silicon each between .5 to 1.0 per cent., nickel 31-34 per cent., chromium 19.5-21.5 per cent., cobalt 28.5-81.5 per cent., tungsten 6.0-7.5 per cent., molybdenum to substantially less than one per cent., columbium 2.753.9 per cent., nitrogen about .15 per cent., iron to and no substantial proportion of any other constituent.

5. A metal alloy especially useful as a composition for structures, such as gas turbines, normally subjected to high temperatures consisting essentially of carbon 0.27-0.30 per cent., small percentages of manganese and silicon, nickel 31 to 35 per cent., chromium 18 to 22 per cent., co-

balt 17 to 20 per cent., tungsten 5.5 to 8 per cent.,

columbium 2-.5 to 4 per cent., nitrogen 0.10 to 0.20

per cent., the remainder substantially all iron. FRANCIS B. FOLEY.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,246,078 Rohn et al. June 17, 1941 2,432,618 Franks et a1 Dec. 16, 1947 OTHER REFERENCES Progress Report on Heat Resisting Metals for GasTurbine Parts (N-102); P. B. No. 39, 579; Sept. 21, 1943; stamped: declassified to open Feb. 18, 1946, page 6. 

1. A METAL ALLOY ESPECIALLY USEFUL AS A COMPOSITION FOR STRUCTURES, SUCH AS GAS TURBINES, NORMALLY SUBJECTED TO HIGH TEMPERATURE CONSISTING ESSENTIALLY OF CARBON NOT EXCEEDING 1.50 PER CENT., SMALL PERCENTAGES OF MAGANESE AND SILICON, NICKEL 30 TO 40 PER CENT., CHROMIUM 18 TO 24 PER CENT., COBALT 17 TO 35 PER CENT., TUNGSTEN 4 TO 8 PER CENT., MOLYBDENUM 0 TO SUBSTANTIALLY LESS THAN ONE PER CENT., COLUMBIUM OVER 2 AND NOT OVER 6 PER CENT., NITROGEN .10 TO .20 PER CENT., THE REMAINDER SUBSTANTIALLY ALL IRON, THE PERCENTAGE OF THE INGREDIENTS OTHER THAN IRON BEING SUCH, WITHIN THE SPECIFIED LIMITS, THAT THE PROPORTION OF IRON DOES NOT EXCEED 25%. 